[mythtv-users] Storing recordings on network share

belcampo belcampo at zonnet.nl
Sat Sep 24 17:37:14 UTC 2011

On 09/23/2011 06:45 PM, Travis Tabbal wrote:
> On Fri, Sep 23, 2011 at 4:48 AM, belcampo <belcampo at zonnet.nl 
> <mailto:belcampo at zonnet.nl>> wrote:
>     What are we talking about, in terms of large buffers, I mean. Even 6
>     concurrent HD-streams of probably <= 20Mb/s only generate 120Mb/s
>     being
>     15MB/s, with todays drives easily being capable of > 80MB/s.
> Not under high random I/O load, which is what we get with Myth when 
> dealing with multiple recordings and such.
If you use xfs with with allocsize=1024M, one can easy avoid most of the 
randomness of videofile-writes and thereby avoiding allmost if not all 
of the mentioned troubles.
> Modern drives, even many SDDs, can't sustain high random I/O loads at 
> even close to 80MB/s. Not unless you go with high-IOPS disks, then you 
> start talking real money. Nothing you would waste on recording TV. The 
> streaming I/O capacity of a drive is meaningless to us, as we never 
> use it. On a mechanical HDD, seek time is close to 10ms, that's 10ms 
> that you can't do I/O. Start doing a bunch of those, and you start 
> seeing slowdowns.  Every block you write, particularly when sync() is 
> being called, means you seek to write it, then seek to the filesystem 
> structures and update those, then seek to do it again.. you quickly 
> get to where you're seeking more than reading/writing. That's where 
> the recommendation comes from to keep the database and the recordings 
> separated, the database can issue a LOT of seek calls.
> And we're dealing with a stream here. A file copy can just wait to 
> read the next bit in, if we don't write fast enough, we corrupt the 
> recording. The codecs can fix some of that, but you drop enough data 
> and you can't really enjoy the recording as it will be all pixelated 
> or choppy.
> For just one example, look at the random I/O test at the bottom of 
> this article:
> http://www.anandtech.com/show/3734/seagates-momentus-xt-review-finally-a-good-hybrid-hdd/3
> From the article:
> "Random read/write performance is abysmal. You can't really make out 
> the numbers here but that's 0.7MB/s for reads and 0.3MB/s for writes 
> compared to 40MB/s+ for the SSDs. It's the poor random access 
> performance that ultimately prevents the Momentus XT from feeling like 
> an SSD most of the time."
> .3MB/s.....
> That's a far rougher test than we would hit our drives with, but it 
> shows the huge performance drop from random I/O. Even the SSDs in 
> their test only got to 40MB/s.
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